It is necessary to test fiber optic cable before installation to determine whether the cable is adequate for the application and free from defects. Testing is accomplished by utilizing a matching launch cable between the test light source and the cable to be tested with a power meter connected to the free end cf the cable under test. The launch cable has to mate with the light source and has to have a connector which is mated to the one on the cable to be tested. Further, the launch cable has to have the same diameter as the cable under test in order to prevent loss from leakage around the mismatched cable diameters. This means that test equipment would have to include at least one launch cable of each different available size: 50, 62.5, 85 and 100 microns. This makes for expensive and cumbersome test equipment. Even with matched diameter cables there is leakage and consequent inaccuracy because of variation within manufacturing tolerances of approximately 3% in diameter and 6% in area. In addition, in multimode fiber optic cable there is typically uneven distribution of light in the cable because of the non-uniform effect of the cable on the different modes of light. Typically the higher order modes travel longer distances than the lower order modes and so the high order modes are more likely to be attenuated, causing a concentration of lower order light in the cable. This creates an inconsistency in test conditions because the actual light distribution from the launch cable is uncertain. While mode scramblers and mode filters are available, they do require matching diameter cables and they are difficult to accurately calibrate for test applications. Inaccuracies are also introduced in fiber optic test equipment due to mismatch in the numerical apertures between the launch cable and test light source when a graded index launch cable is used. This is so because graded index launch cables transmit the light source output with all of its peculiarities and therefore preserve all of the variations in light sources due to manufacturing variations.
A separate problem occurs because of the field personnel's difficulty in obtaining accurate readings due to a misalignment of the launch cable and the cable to be tested or because of differences in diameter in those cables. The difference in diameters may be due merely to manufacturing tolerances or to the use of mismatched cables. The result is that the loss of light at the launch cable/test cable interface requires an arithmetic accommodation of the power meter reading in order to identify the loss attributable only to the cable under test.